1. Field of the Invention
The present invention relates to a supersonic microscope intended to scan that face of a sample which is to be viewed through the microscope with supersonic beam and, more particularly, it relates to a supersonic microscope intended to use, as the coupler liquid, a low temperature liquid such as nitrogen, argon and helium liquids.
2. Description of the Related Art
The supersonic microscope which has been recently developed attracts attention these days as a third microscope following the optical and electronic ones. The supersonic microscope can obtain the image of a sample in such a way that supersonic beam is focused onto the viewed face of the sample by means of an acoustic lens to two-dimensionally scan the face of the sample. When a supersonic wave of high frequencies (ranging from several hundreds MHz to several GHz) is used to increase resolving power in the case of this microscope, the interval between the acoustic lens and the sample must be made smaller as the frequency of the supersonic wave becomes higher because the acoustic absorption ratio of coupler liquid (or water) is proportional to the square of frequency. The interval between the acoustic lens and the sample is defined by the curvature radius of the acoustic lens. An acoustic lens having a small radius of curvature must be used to shorten the interval. In a case where the frequency of supersonic wave used is several GHz, for example, the curvature radius of acoustic lens must be made smaller than several tens .mu.m. It is extremely difficult to make an acoustic lens which has such a finely curved surface. Even if such acoustic lens can be made, it will raise the cost of the microscope.
To enhance resolving power without setting the curvature radius of acoustic lens to the above-mentioned value, there has been provided another supersonic microscope (which will be hereinafter referred to as a low temperature supersonic microscope) intended to use, as coupler liquid, a low temperature liquid having a lower sound speed than that of water, such as nitrogen, argon and helium liquids and to scan the viewed face of sample in this low temperature liquid. The supersonic microscope of this type disclosed in U.S. Pat. Ser. No. 235,544, for example, temporarily raises the pressure of a container in which the low temperature liquid is contained to prevent the image of a sample from being blurred by the boiling of the low temperature liquid in the course of scanning or viewing the sample. Even when the image of the sample is created in this manner, it has still blurred portions. This is because the surface of the low temperature liquid which is under pressure is rippled by those vibrations which are caused by the acoustic lens scanning in directions X and Y and also caused by outside factors. Therefore, images of samples viewed through the conventional supersonic microscopes were not quite but still a little blurred.